Multi-site Emails not sending

Hi all, I've just signed up with WPMU with the grand ambition of running a multisite network for my site.

I seem to have successfully activated the multisite network, and using subdirectories have activated a couple of test sites. However, the emails that go out to users are not being received.

A few months prior I noticed that emails from Contact Form 7 were suddenly not working either, so I suspect this isn't so much a multisite problem as a larger problem with our server.

Can anyone point me in the right direction for solving this problem? Not going to get very far with this network if I can't email the users and they can't get emails from their readers....

Thanks, I'm glad to be here at WMPU. Lots to learn, excited to grow.